This invention relates to an electronic musical instrument and specifically relates to an improvement in a variable delay echo-machine which is a derivative of an echo-machine defined as a sound system designed to produce echo effects by miximg a portion of the output signal of a delay element the input signal of the same delay element and which is designed to produce more impressive and fantastic sound effects by variation of the magnitude of the delay time produced by the delay element, a major element of the aforementioned echo-machine.
Referring to FIG. 1 illustrating a block diagram of a variable delay echo-machine in the prior art, an input signal representing a sound signal received at an input signal terminal 1 is applied to a delay element 4 composed of memory elements e.g. BBDs, CCDs or transistors, through a first low pass filter 2 which is to eliminate erroneous signals to be discussed later and a mixer 3 which mixes a portion of the output signal of the delay element 4 from a signal feedback line 5 back to the input signal of the delay element 4. The shift pulses for shifting the contents of the memory elements composing the delay element 4 are produced by a shift pulse generator 6 containing a variable frequency oscillation circuit designed for varying the output frequency thereof in accordance with the shift pulse frequency regulation signals to be received at terminal 8 and are supplied to the delay element 4 through a shift pulse feed line 7. The final output of the delay element 4 is produced from an output signal terminal 10, after passing through a second low pass filter 9 which is to eliminate the shift pulse frequency components which have adverse effects discussed later.
It is well-known, in the prior art, that the aforementioned variable delay echo machine is effective to produce a type of sound effects which resembles those produced by a chorus. Further, it is also well-known, in the prior art, that a type of impressive and fantastic sound effect, more specifically the stereophonic effects giving the audience the impression as if the sound originates from a moving sound source, can be readily produced by the application to the terminal 8 of extremely low frequency signals, such as functional waves produced by extremely low frequency oscillators, arbitrary waves produced by manual operations and specific waves formed from the envelopes of sound signal waves, because each of them causes the shift pulse generator 6 to generate shift pulses with a frequency determined by the nature of the specific extremely low frequency signal.
The U.S. Pat. No. 3,895,553 discloses an invention in which the frequency of the shift pulses is varied in a variable delay echo-machine similar to that illustrated in FIG. 1 to vary the magnitude of the delay time, while the input sound signal is sampled according to a fixed sampling frequency, before being applied to a delay element composed of analog shift registers. Japanese Patent Application No. 47-73189, filed July 20, 1972 and laid open Mar. 25, 1974, which is utilized as the basis of the right of priority for aforementioned U.S. Pat. No. 3,895,553 discloses another invention in which a portion of the sound signal delayed with a variable magnitude by the delay element is mixed with the original sound signal by an electronic means. Further, it is widely known in the prior art that an active filter composed of a combination of integrating circuits and operational amplifiers works as a low pass filter with variable cut-off frequency as discussed later and that a variation of the electrical capacitance of capacitors included in the integrating circuits allows such an active filter to vary its cut-off frequency. The U.S. Pat. No. 3,701,059 discloses an invention relating to a low pass filter with a variable cut-off frequency which is capable of varying the cut-off frequency thereof in proportion to the duty ratio of a received intermittent input signal applied to the signal input terminal.
Referring to FIG. 2 illustrating the amplitude-time relations for explaining the principle accounting for the occurrence of erroneous signals in the apparatus illustrated in FIG. 1, in the event the frequency of the signal A is close to that of the shift pulse train B, an erroneous signal D shown in a dotted line could be produced at the output signal terminal 10. The process in which such an erroneous signal occurs is as follows:
(i) The input sound signal A is sampled by one of the shift pulses B at the trailing edge thereof. PA0 (ii) The amplitude of the sound signal sampled by the above means is stored in the first stage of the memory elements included in the delay element during one cycle period of the shift pulse train. PA0 (iii) The succeeding shift pulse shifts the stored content of the first stage to the second stage of the memory elements, samples the sound signal and stores the amplitude of the sound signal sampled by the trailing edge of the shift pulse in the first stage of the memory elements during the next cycle period of the shift pulse train. PA0 (iv) Therefore, the amplitude stored in the first stage is represented by a staircase wave. Further, the output signal wave appearing at the output terminal 10 is represented by a staircase wave which lags by the period which is the product of the number of stages of the memory elements and the cycle period of the shift pulse train. PA0 (v) In the event the frequency of the shift pulse train B is close to that of the sound signal A, it is difficult to sample the sound signal twice during one cycle period of the sound signal, and this causes it to be impossible to reproduce the input signal from the envelop of the staircase wave appearing at the output terminal 10.
This is a problem, because it tends to be difficult to distinguish the sound signal A from the erroneous signal D. To prevent this undesirable phenomena, it is commonly practiced in the prior art to arrange a first low pass filter 2 with a cut-off frequency which is less than half of the shift pulse frequency between the input signal terminal 1 and the mixer 3.
On the other hand, referring to FIG. 3 illustrating the amplitude-time relations for explaining the causes of inclusion of the shift pulse frequency components in the sound signal of the apparatus illustrated in FIG. 1, when an input sound signal E is shifted by a shift pulse train F, the delay element 4 produces a signal G in the form of staircase wave. This means that the output of the delay element 4 contains components of the shift pulse frequency. To prevent this type of noise components from mixing with the sound signals, it is commonly practiced in the prior art to arrange a second low pass filter 9 with a cut-off frequency which is less than a half of the shift pulse frequency between the delay element 4 and the output signal terminal 10.
However, when these two low pass filters 2 and 9 are employed for elimination of erroneous signals and shift pulse frequency components, respectively, attention must be paid to the phenomena that a variation in shift pulse frequency results in a variation in the frequency of the erroneous signals and of the shift pulse frequency components. In other words, the lowest value of the shift pulse frequency determines the lowest value of the frequency of the erroneous signals and of the shift pulse frequency components. Therefore, in order to be effective even for the case of the lowest value of the shift pulse frequency, the cut-off frequency of these two low pass filters 2 and 9 must be selected to be less one half than the lowest possible value of the shift pulse frequency to be adopted in the delay element 4.
This restrictive requirement for selection of the cut-off frequency of the two low pass filters having a low fixed cut-off frequency brings about such a big problem as to degrade the fidelity of sound signals, because the lower cut-off frequency results in a narrow pass band for the entire system particularly under the conditions that the frequency of the shift pulses turns out to be very high.